/**
 * @file
 * DNS - host name to IP address resolver.
 *
 */

/**

 * This file implements a DNS host name to IP address resolver.

 * Port to lwIP from uIP
 * by Jim Pettinato April 2007

 * uIP version Copyright (c) 2002-2003, Adam Dunkels.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * DNS.C
 *
 * The lwIP DNS resolver functions are used to lookup a host name and
 * map it to a numerical IP address. It maintains a list of resolved
 * hostnames that can be queried with the dns_lookup() function.
 * New hostnames can be resolved using the dns_query() function.
 *
 * The lwIP version of the resolver also adds a non-blocking version of
 * gethostbyname() that will work with a raw API application. This function
 * checks for an IP address string first and converts it if it is valid.
 * gethostbyname() then does a dns_lookup() to see if the name is 
 * already in the table. If so, the IP is returned. If not, a query is 
 * issued and the function returns with a ERR_INPROGRESS status. The app
 * using the dns client must then go into a waiting state.
 *
 * Once a hostname has been resolved (or found to be non-existent),
 * the resolver code calls a specified callback function (which 
 * must be implemented by the module that uses the resolver).
 */

/*-----------------------------------------------------------------------------
 * RFC 1035 - Domain names - implementation and specification
 * RFC 2181 - Clarifications to the DNS Specification
 *----------------------------------------------------------------------------*/

/** @todo: define good default values (rfc compliance) */
/** @todo: improve answer parsing, more checkings... */
/** @todo: check RFC1035 - 7.3. Processing responses */

/*-----------------------------------------------------------------------------
 * Includes
 *----------------------------------------------------------------------------*/

#include "lwip/opt.h"

#if LWIP_DNS			/* don't build if not configured for use in lwipopts.h */

#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/dns.h"

#include <string.h>

/** DNS server IP address */
#ifndef DNS_SERVER_ADDRESS
#define DNS_SERVER_ADDRESS        inet_addr("208.67.222.222")	/* resolver1.opendns.com */
#endif

/** DNS server port address */
#ifndef DNS_SERVER_PORT
#define DNS_SERVER_PORT           53
#endif

/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
#define DNS_MAX_RETRIES           4
#endif

/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
#define DNS_MAX_TTL               604800
#endif

/* DNS protocol flags */
#define DNS_FLAG1_RESPONSE        0x80
#define DNS_FLAG1_OPCODE_STATUS   0x10
#define DNS_FLAG1_OPCODE_INVERSE  0x08
#define DNS_FLAG1_OPCODE_STANDARD 0x00
#define DNS_FLAG1_AUTHORATIVE     0x04
#define DNS_FLAG1_TRUNC           0x02
#define DNS_FLAG1_RD              0x01
#define DNS_FLAG2_RA              0x80
#define DNS_FLAG2_ERR_MASK        0x0f
#define DNS_FLAG2_ERR_NONE        0x00
#define DNS_FLAG2_ERR_NAME        0x03

/* DNS protocol states */
#define DNS_STATE_UNUSED          0
#define DNS_STATE_NEW             1
#define DNS_STATE_ASKING          2
#define DNS_STATE_DONE            3

#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS message header */
  struct dns_hdr
{
  u16_t id;
  u8_t flags1;
  u8_t flags2;
  u16_t numquestions;
  u16_t numanswers;
  u16_t numauthrr;
  u16_t numextrarr;
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
  PACK_STRUCT_BEGIN
/** DNS query message structure */
  struct dns_query
{
  /* DNS query record starts with either a domain name or a pointer
     to a name already present somewhere in the packet. */
  u16_t type;
  u16_t class;
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
  PACK_STRUCT_BEGIN
/** DNS answer message structure */
  struct dns_answer
{
  /* DNS answer record starts with either a domain name or a pointer
     to a name already present somewhere in the packet. */
  u16_t type;
  u16_t class;
  u32_t ttl;
  u16_t len;
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
/** DNS table entry */
  struct dns_table_entry
{
  u8_t state;
  u8_t numdns;
  u8_t tmr;
  u8_t retries;
  u8_t seqno;
  u8_t err;
  u32_t ttl;
  char name[DNS_MAX_NAME_LENGTH];
  struct ip_addr ipaddr;
  /* pointer to callback on DNS query done */
  dns_found_callback found;
  void *arg;
};


/* forward declarations */
static void dns_recv (void *s, struct udp_pcb *pcb, struct pbuf *p,
		      struct ip_addr *addr, u16_t port);
static void dns_check_entries (void);

/*-----------------------------------------------------------------------------
 * Globales
 *----------------------------------------------------------------------------*/

/* DNS variables */
static struct udp_pcb *dns_pcb;
static u8_t dns_seqno;
static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
static struct ip_addr dns_servers[DNS_MAX_SERVERS];

#if (DNS_USES_STATIC_BUF == 1)
static u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 1) */

/**
 * Initialize the resolver: set up the UDP pcb and configure the default server
 * (DNS_SERVER_ADDRESS).
 */
void
dns_init ()
{
  struct ip_addr dnsserver;

  /* initialize default DNS server address */
  dnsserver.addr = DNS_SERVER_ADDRESS;

  LWIP_DEBUGF (DNS_DEBUG, ("dns_init: initializing\n"));

  /* if dns client not yet initialized... */
  if (dns_pcb == NULL)
    {
      dns_pcb = udp_new ();

      if (dns_pcb != NULL)
	{
	  /* initialize DNS table not needed (initialized to zero since it is a
	   * global variable) */
	  LWIP_ASSERT
	    ("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
	     DNS_STATE_UNUSED == 0);

	  /* initialize DNS client */
	  udp_bind (dns_pcb, IP_ADDR_ANY, 0);
	  udp_recv (dns_pcb, dns_recv, NULL);

	  /* initialize default DNS primary server */
	  dns_setserver (0, &dnsserver);
	}
    }
}

/**
 * Initialize one of the DNS servers.
 *
 * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
 * @param dnsserver IP address of the DNS server to set
 */
void
dns_setserver (u8_t numdns, struct ip_addr *dnsserver)
{
  if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
      (dnsserver != NULL) && (dnsserver->addr != 0))
    {
      dns_servers[numdns] = (*dnsserver);
    }
}

/**
 * Obtain one of the currently configured DNS server.
 *
 * @param numdns the index of the DNS server
 * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
 *         server has not been configured.
 */
struct ip_addr
dns_getserver (u8_t numdns)
{
  if (numdns < DNS_MAX_SERVERS)
    {
      return dns_servers[numdns];
    }
  else
    {
      return *IP_ADDR_ANY;
    }
}

/**
 * The DNS resolver client timer - handle retries and timeouts and should
 * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
 */
void
dns_tmr (void)
{
  if (dns_pcb != NULL)
    {
      LWIP_DEBUGF (DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
      dns_check_entries ();
    }
}

/**
 * Look up a hostname in the array of known hostnames.
 *
 * @note This function only looks in the internal array of known
 * hostnames, it does not send out a query for the hostname if none
 * was found. The function dns_enqueue() can be used to send a query
 * for a hostname.
 *
 * @param name the hostname to look up
 * @return the hostname's IP address, as u32_t (instead of struct ip_addr to
 *         better check for failure: != 0) or 0 if the hostname was not found
 *         in the cached dns_table.
 */
static u32_t
dns_lookup (const char *name)
{
  u8_t i;

  /* Walk through name list, return entry if found. If not, return NULL. */
  for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
      if ((dns_table[i].state == DNS_STATE_DONE) &&
	  (strcmp (name, dns_table[i].name) == 0))
	{
	  LWIP_DEBUGF (DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
	  ip_addr_debug_print (DNS_DEBUG, &(dns_table[i].ipaddr));
	  LWIP_DEBUGF (DNS_DEBUG, ("\n"));
	  return dns_table[i].ipaddr.addr;
	}
    }

  return 0;
}

#if DNS_DOES_NAME_CHECK
/**
 * Compare the "dotted" name "query" with the encoded name "response"
 * to make sure an answer from the DNS server matches the current dns_table
 * entry (otherwise, answers might arrive late for hostname not on the list
 * any more).
 *
 * @param query hostname (not encoded) from the dns_table
 * @param response encoded hostname in the DNS response
 * @return 0: names equal; 1: names differ
 */
static u8_t
dns_compare_name (unsigned char *query, unsigned char *response)
{
  unsigned char n;

  do
    {
      n = *response++;
    /** @see RFC 1035 - 4.1.4. Message compression */
      if ((n & 0xc0) == 0xc0)
	{
	  /* Compressed name */
	  break;
	}
      else
	{
	  /* Not compressed name */
	  while (n > 0)
	    {
	      if ((*query) != (*response))
		{
		  return 1;
		}
	      ++response;
	      ++query;
	      --n;
	    };
	  ++query;
	}
    }
  while (*response != 0);

  return 0;
}
#endif /* DNS_DOES_NAME_CHECK */

/**
 * Walk through a compact encoded DNS name and return the end of the name.
 *
 * @param query encoded DNS name in the DNS server response
 * @return end of the name
 */
static unsigned char *
dns_parse_name (unsigned char *query)
{
  unsigned char n;

  do
    {
      n = *query++;
    /** @see RFC 1035 - 4.1.4. Message compression */
      if ((n & 0xc0) == 0xc0)
	{
	  /* Compressed name */
	  break;
	}
      else
	{
	  /* Not compressed name */
	  while (n > 0)
	    {
	      ++query;
	      --n;
	    };
	}
    }
  while (*query != 0);

  return query + 1;
}

/**
 * Send a DNS query packet.
 *
 * @param numdns index of the DNS server in the dns_servers table
 * @param name hostname to query
 * @param id index of the hostname in dns_table, used as transaction ID in the
 *        DNS query packet
 * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
 */
static err_t
dns_send (u8_t numdns, const char *name, u8_t id)
{
  err_t err;
  struct dns_hdr *hdr;
  struct dns_query qry;
  struct pbuf *p;
  char *query, *nptr;
  const char *pHostname;
  u8_t n;

  LWIP_DEBUGF (DNS_DEBUG,
	       ("dns_send: dns_servers[%" U16_F "] \"%s\": request\n",
		(u16_t) (numdns), name));
  LWIP_ASSERT ("dns server out of array", numdns < DNS_MAX_SERVERS);
  LWIP_ASSERT ("dns server has no IP address set",
	       dns_servers[numdns].addr != 0);

  /* if here, we have either a new query or a retry on a previous query to process */
  p =
    pbuf_alloc (PBUF_TRANSPORT,
		sizeof (struct dns_hdr) + DNS_MAX_NAME_LENGTH +
		sizeof (struct dns_query), PBUF_RAM);
  if (p != NULL)
    {
      LWIP_ASSERT ("pbuf must be in one piece", p->next == NULL);
      /* fill dns header */
      hdr = (struct dns_hdr *) p->payload;
      memset (hdr, 0, sizeof (struct dns_hdr));
      hdr->id = htons (id);
      hdr->flags1 = DNS_FLAG1_RD;
      hdr->numquestions = htons (1);
      query = (char *) hdr + sizeof (struct dns_hdr);
      pHostname = name;
      --pHostname;

      /* convert hostname into suitable query format. */
      do
	{
	  ++pHostname;
	  nptr = query;
	  ++query;
	  for (n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname)
	    {
	      *query = *pHostname;
	      ++query;
	      ++n;
	    }
	  *nptr = n;
	}
      while (*pHostname != 0);
      *query++ = '\0';

      /* fill dns query */
      qry.type = htons (DNS_RRTYPE_A);
      qry.class = htons (DNS_RRCLASS_IN);
      MEMCPY (query, &qry, sizeof (struct dns_query));

      /* resize pbuf to the exact dns query */
      pbuf_realloc (p,
		    (query + sizeof (struct dns_query)) -
		    ((char *) (p->payload)));

      /* connect to the server for faster receiving */
      udp_connect (dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
      /* send dns packet */
      err = udp_sendto (dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);

      /* free pbuf */
      pbuf_free (p);
    }
  else
    {
      err = ERR_MEM;
    }

  return err;
}

/**
 * dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void
dns_check_entry (u8_t i)
{
  struct dns_table_entry *pEntry = &dns_table[i];

  LWIP_ASSERT ("array index out of bounds", i < DNS_TABLE_SIZE);

  switch (pEntry->state)
    {

    case DNS_STATE_NEW:
      {
	/* initialize new entry */
	pEntry->state = DNS_STATE_ASKING;
	pEntry->numdns = 0;
	pEntry->tmr = 1;
	pEntry->retries = 0;

	/* send DNS packet for this entry */
	dns_send (pEntry->numdns, pEntry->name, i);
	break;
      }

    case DNS_STATE_ASKING:
      {
	if (--pEntry->tmr == 0)
	  {
	    if (++pEntry->retries == DNS_MAX_RETRIES)
	      {
		if ((pEntry->numdns + 1 < DNS_MAX_SERVERS)
		    && (dns_servers[pEntry->numdns + 1].addr != 0))
		  {
		    /* change of server */
		    pEntry->numdns++;
		    pEntry->tmr = 1;
		    pEntry->retries = 0;
		    break;
		  }
		else
		  {
		    LWIP_DEBUGF (DNS_DEBUG,
				 ("dns_check_entry: \"%s\": timeout\n",
				  pEntry->name));
		    /* call specified callback function if provided */
		    if (pEntry->found)
		      (*pEntry->found) (pEntry->name, NULL, pEntry->arg);
		    /* flush this entry */
		    pEntry->state = DNS_STATE_UNUSED;
		    pEntry->found = NULL;
		    break;
		  }
	      }

	    /* wait longer for the next retry */
	    pEntry->tmr = pEntry->retries;

	    /* send DNS packet for this entry */
	    dns_send (pEntry->numdns, pEntry->name, i);
	  }
	break;
      }

    case DNS_STATE_DONE:
      {
	/* if the time to live is nul */
	if (--pEntry->ttl == 0)
	  {
	    LWIP_DEBUGF (DNS_DEBUG,
			 ("dns_check_entry: \"%s\": flush\n", pEntry->name));
	    /* flush this entry */
	    pEntry->state = DNS_STATE_UNUSED;
	    pEntry->found = NULL;
	  }
	break;
      }
    case DNS_STATE_UNUSED:
      /* nothing to do */
      break;
    default:
      LWIP_ASSERT ("unknown dns_table entry state:", 0);
      break;
    }
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void
dns_check_entries (void)
{
  u8_t i;

  for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
      dns_check_entry (i);
    }
}

/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 *
 * @params see udp.h
 */
static void
dns_recv (void *arg, struct udp_pcb *pcb, struct pbuf *p,
	  struct ip_addr *addr, u16_t port)
{
  u8_t i;
  char *pHostname;
  struct dns_hdr *hdr;
  struct dns_answer ans;
  struct dns_table_entry *pEntry;
  u8_t nquestions, nanswers;
#if (DNS_USES_STATIC_BUF == 0)
  u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 0) */
#if (DNS_USES_STATIC_BUF == 2)
  u8_t *dns_payload;
#endif /* (DNS_USES_STATIC_BUF == 2) */

  LWIP_UNUSED_ARG (arg);
  LWIP_UNUSED_ARG (pcb);
  LWIP_UNUSED_ARG (addr);
  LWIP_UNUSED_ARG (port);

  /* is the dns message too big ? */
  if (p->tot_len > DNS_MSG_SIZE)
    {
      LWIP_DEBUGF (DNS_DEBUG, ("dns_recv: pbuf too big\n"));
      /* free pbuf and return */
      goto memerr1;
    }

  /* is the dns message big enough ? */
  if (p->tot_len <
      (sizeof (struct dns_hdr) + sizeof (struct dns_query) +
       sizeof (struct dns_answer)))
    {
      LWIP_DEBUGF (DNS_DEBUG, ("dns_recv: pbuf too small\n"));
      /* free pbuf and return */
      goto memerr1;
    }

#if (DNS_USES_STATIC_BUF == 2)
  dns_payload = mem_malloc (p->tot_len);
  if (dns_payload == NULL)
    {
      LWIP_DEBUGF (DNS_DEBUG, ("dns_recv: mem_malloc error\n"));
      /* free pbuf and return */
      goto memerr1;
    }
#endif /* (DNS_USES_STATIC_BUF == 2) */

  /* copy dns payload inside static buffer for processing */
  if (pbuf_copy_partial (p, dns_payload, p->tot_len, 0) == p->tot_len)
    {
      /* The ID in the DNS header should be our entry into the name table. */
      hdr = (struct dns_hdr *) dns_payload;
      i = htons (hdr->id);
      if (i < DNS_TABLE_SIZE)
	{
	  pEntry = &dns_table[i];
	  if (pEntry->state == DNS_STATE_ASKING)
	    {
	      /* This entry is now completed. */
	      pEntry->state = DNS_STATE_DONE;
	      pEntry->err = hdr->flags2 & DNS_FLAG2_ERR_MASK;

	      /* We only care about the question(s) and the answers. The authrr
	         and the extrarr are simply discarded. */
	      nquestions = htons (hdr->numquestions);
	      nanswers = htons (hdr->numanswers);

	      /* Check for error. If so, call callback to inform. */
	      if (((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0)
		  || (pEntry->err != 0) || (nquestions != 1))
		{
		  LWIP_DEBUGF (DNS_DEBUG,
			       ("dns_recv: \"%s\": error in flags\n",
				pEntry->name));
		  /* call callback to indicate error, clean up memory and return */
		  goto responseerr;
		}

#if DNS_DOES_NAME_CHECK
	      /* Check if the name in the "question" part match with the name in the entry. */
	      if (dns_compare_name
		  ((unsigned char *) (pEntry->name),
		   (unsigned char *) dns_payload + sizeof (struct dns_hdr)) !=
		  0)
		{
		  LWIP_DEBUGF (DNS_DEBUG,
			       ("dns_recv: \"%s\": response not match to query\n",
				pEntry->name));
		  /* call callback to indicate error, clean up memory and return */
		  goto responseerr;
		}
#endif /* DNS_DOES_NAME_CHECK */

	      /* Skip the name in the "question" part */
	      pHostname =
		(char *) dns_parse_name ((unsigned char *) dns_payload +
					 sizeof (struct dns_hdr)) +
		sizeof (struct dns_query);

	      while (nanswers > 0)
		{
		  /* skip answer resource record's host name */
		  pHostname =
		    (char *) dns_parse_name ((unsigned char *) pHostname);

		  /* Check for IP address type and Internet class. Others are discarded. */
		  MEMCPY (&ans, pHostname, sizeof (struct dns_answer));
		  if ((ntohs (ans.type) == DNS_RRTYPE_A)
		      && (ntohs (ans.class) == DNS_RRCLASS_IN)
		      && (ntohs (ans.len) == sizeof (struct ip_addr)))
		    {
		      /* read the answer resource record's TTL, and maximize it if needed */
		      pEntry->ttl = ntohl (ans.ttl);
		      if (pEntry->ttl > DNS_MAX_TTL)
			{
			  pEntry->ttl = DNS_MAX_TTL;
			}
		      /* read the IP address after answer resource record's header */
		      MEMCPY (&(pEntry->ipaddr),
			      (pHostname + sizeof (struct dns_answer)),
			      sizeof (struct ip_addr));
		      LWIP_DEBUGF (DNS_DEBUG,
				   ("dns_recv: \"%s\": response = ",
				    pEntry->name));
		      ip_addr_debug_print (DNS_DEBUG, (&(pEntry->ipaddr)));
		      LWIP_DEBUGF (DNS_DEBUG, ("\n"));
		      /* call specified callback function if provided */
		      if (pEntry->found)
			{
			  (*pEntry->found) (pEntry->name, &pEntry->ipaddr,
					    pEntry->arg);
			}
		      /* deallocate memory and return */
		      goto memerr2;
		    }
		  else
		    {
		      pHostname =
			pHostname + sizeof (struct dns_answer) +
			htons (ans.len);
		    }
		  --nanswers;
		}
	      LWIP_DEBUGF (DNS_DEBUG,
			   ("dns_recv: \"%s\": error in response\n",
			    pEntry->name));
	      /* call callback to indicate error, clean up memory and return */
	      goto responseerr;
	    }
	}
    }

  /* deallocate memory and return */
  goto memerr2;

responseerr:
  /* ERROR: call specified callback function with NULL as name to indicate an error */
  if (pEntry->found)
    {
      (*pEntry->found) (pEntry->name, NULL, pEntry->arg);
    }
  /* flush this entry */
  pEntry->state = DNS_STATE_UNUSED;
  pEntry->found = NULL;

memerr2:
#if (DNS_USES_STATIC_BUF == 2)
  /* free dns buffer */
  mem_free (dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */

memerr1:
  /* free pbuf */
  pbuf_free (p);
  return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param found a callback founction to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return @return a err_t return code.
 */
static err_t
dns_enqueue (const char *name, dns_found_callback found, void *callback_arg)
{
  u8_t i;
  u8_t lseq, lseqi;
  struct dns_table_entry *pEntry = NULL;

  /* search an unused entry, or the oldest one */
  lseq = lseqi = 0;
  for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
      pEntry = &dns_table[i];
      /* is it an unused entry ? */
      if (pEntry->state == DNS_STATE_UNUSED)
	break;

      /* check if this is the oldest completed entry */
      if (pEntry->state == DNS_STATE_DONE)
	{
	  if ((dns_seqno - pEntry->seqno) > lseq)
	    {
	      lseq = dns_seqno - pEntry->seqno;
	      lseqi = i;
	    }
	}
    }

  /* if we don't have found an unused entry, use the oldest completed one */
  if (i == DNS_TABLE_SIZE)
    {
      if ((lseqi >= DNS_TABLE_SIZE)
	  || (dns_table[lseqi].state != DNS_STATE_DONE))
	{
	  /* no entry can't be used now, table is full */
	  LWIP_DEBUGF (DNS_DEBUG,
		       ("dns_enqueue: \"%s\": DNS entries table is full\n",
			name));
	  return ERR_MEM;
	}
      else
	{
	  /* use the oldest completed one */
	  i = lseqi;
	  pEntry = &dns_table[i];
	}
    }

  /* use this entry */
  LWIP_DEBUGF (DNS_DEBUG,
	       ("dns_enqueue: \"%s\": use DNS entry %" U16_F "\n", name,
		(u16_t) (i)));

  /* fill the entry */
  pEntry->state = DNS_STATE_NEW;
  pEntry->seqno = dns_seqno++;
  pEntry->found = found;
  pEntry->arg = callback_arg;
  strcpy (pEntry->name, name);

  /* force to send query without waiting timer */
  dns_check_entry (i);

  /* dns query is enqueued */
  return ERR_INPROGRESS;
}

/**
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a struct ip_addr where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t
dns_gethostbyname (const char *hostname, struct ip_addr * addr,
		   dns_found_callback found, void *callback_arg)
{
  /* not initialized or no valid server yet, or invalid addr pointer
   * or invalid hostname or invalid hostname length */
  if ((dns_pcb == NULL) || (addr == NULL) ||
      (!hostname) || (!hostname[0]) ||
      (strlen (hostname) >= DNS_MAX_NAME_LENGTH))
    {
      return ERR_VAL;
    }

#if LWIP_HAVE_LOOPIF
  if (strcmp (hostname, "localhost") == 0)
    {
      addr->addr = INADDR_LOOPBACK;
      return ERR_OK;
    }
#endif /* LWIP_HAVE_LOOPIF */

  /* host name already in octet notation? set ip addr and return ERR_OK
   * already have this address cached? */
  if (((addr->addr = inet_addr (hostname)) != INADDR_NONE) ||
      ((addr->addr = dns_lookup (hostname)) != 0))
    {
      return ERR_OK;
    }

  /* queue query with specified callback */
  return dns_enqueue (hostname, found, callback_arg);
}

#endif /* LWIP_DNS */
